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The absolute iodine number follows the same course as the relative one
being very high in the drying oils (over 150) and the semi-drying oils (120-155)
and lower in the non-drying oils (about ico). For animal fats the number is
usually 90-ico, but in rare cases slightly exceeds ico (in some American lards)
This number is of especial importance in the analysis of lard and its substitutes
(see Lard).

14. Insoluble, Fixed Fatty Acid Number
(Hehner Number)

This number represents the quantity of non-volatile fatty acids insoluble
in water contained in 100 grams of a fatty substance. Its determination
is carried out as follows :

5 grams of the fat, weighed into a conical flask, are treated with 50
c.c. of 90% alcohol and 5 c.c. of 50% caustic potash solution. The flask
is closed with a stopper through which passes a long glass tube to serve as
reflux condenser and is then heated on a water-bath with frequent shaking
until saponification is complete. The stopper and tube are then removed
and the alcohol distilled of, the last traces being expelled from the open
flask in a boiling water-bath. The soap is dissolved in 150 c.c. of hot water
and then decomposed with a slight excess of dilute sulphuric acid. The
flask is then left on the water-bath until the fatty acids have collected in
a homogeneous layer at the surface, after which it is allowed to cool some-
what and kept in cold water (at 10-15) for about an hour, so that the fatty
acids set to a solid mass. The aqueous liquid is then filtered through a
smooth, thick paper filter, previously dried at 100 and weighed in a weighing
bottle. A further quantity of 200 c.c. of hot water is added to the flask,
which is shaken, left on the water-bath for 15 minutes, allowed to cool
somewhat and placed in cold water, the aqueous liquid being filtered as
before. This treatment is repeated five or six times until the filtrate no
longer reddens a litmus paper immersed in it for 10 minutes. The fatty
acids are then completely melted by addition of a little boiling water and
the whole transferred to the filter, the flask being freed from the last traces
of the fatty acids by several small quantities of hot water, and care taken
that a few drops of water always remain under the acids on the filter. When
all the acids are on the filter, all the water on the latter is allowed to flow
away and the filter immediately placed carefully in the weighing bottle,
which is dried at 100 and weighed at the end of each half-hour until the
difference between two successive weighings is less than I milligram. The
weight of the fatty acids thus obtained, calculated for 100 grams of sub-
stance, gives the insoluble fixed acid number sought.1

The insoluble fatty acid number varies little for most oils and fats, being
usually 95-96-5 for oils and 94-5-97 for fats. Some vegetable and animal oils
and fats are, however, exceptional, especially if they are rich in volatile or

1 With highly altered fats, which may contain appreciable proportions of hydroxy-
acids, or with fats which may be mixed with gummy or gelatinous substances, etc.
(such as sanse oils or bone oils), it is well to treat the insoluble fatty acids with cold
carbon disulpmde or petroleum ether to remove hydroxy-acids and other extraneous
substances (Gianoli: Ann, Soc. chim. de Milano, 1902 p 155)taken, provided t hat these do not differ greatly.quid remains clear.r.